288 ANIMAL LIFE 



drainage basin with another. An example of this is found 

 in Lava Creek in Yellowstone Park. Above Undine and 

 AVraith Falls, both insurmountable, are found an abun- 

 dance of trout. A marsh dry in summer connects Lava 

 Creek with Black Tail Deer Creek, a tributary of the 

 Yellowstone and without waterfall. From the Yellow- 

 stone through this creek and marsh the trout find their 

 way into Lava Creek. In California numerous anomalies 

 have been noted, as the occurrence of Tahoe trout in 

 Feather Eiver and in the Blue Lakes of Amador, which are 

 on the other side of the main crest of the Sierra Nevada 

 from Lake Tahoe, and the occurrence of the Whitney 

 golden trout in Lone Pine Creek, another similar instance. 

 In each case naturalists have found the man who actually 

 carried the species across the divide. If this matter had 

 been investigated a generation later, these cases would have 

 been unexplainable anomalies in geographical distribution. 

 Real causes are almost always simple when they are once 

 known. 



The ways in which species may cross barriers in a state 

 of Nature are as varied as the creatures themselves, and far 

 more varied than the actual barriers. By the long-con- 

 tinued process of adjustment to conditions with the inces- 

 sant destruction of the unadapted, the various organisms 

 have become so well fitted to their surroundings that the 

 casual observer may well suppose that each inhabits the 

 region best fitted for it. Men have even thought that the 

 conditions of life have been fitted to the creatures them- 

 selves, so perfect is this relation. 



155. Character of barriers to distribution. Taking the 

 animal kingdom as a whole, the two great barriers modify- 

 ing distribution are the presence of the sea and changes in 

 temperature. It is only in rare cases that any land ani- 

 mals can cross either of the great oceans, and these rare 

 cases relate chiefly to the arctic regions. For this reason 

 the land fauna? of Africa, South America, and Australia 



